Who overheard the tucks secret?

In Natalie Babbitt's novel "Tuck Everlasting," the character who overhears the Tuck family's secret is a young girl named Winnie Foster. While exploring the woods, she encounters Jesse Tuck and learns about the family's immortality due to drinking from a magical spring. This revelation sets off a series of events that explore themes of life, death, and the consequences of eternal life.